The basic function of a ski boot is to transmit a skier's movements to the skis. Previously, skiers wore everyday winter footwear, and though warm and comfortable, this everyday winter footwear provided little ankle support and gave the skier absolutely no control over the skis. Thick, heavy leather boots supplanted these boots. Steel shanks were installed in the leather sole to counteract buckling effects. Added rigidity was provided with long straps of leather. Porous rubber boot inserts were designed to support the ankle while providing a more conformable fit. Buckle boots implemented with these boots deformed the leather. Plastic ski boots were introduced with forward flex, lateral rigidity, and a height to permit the lower leg to help control skiing.
Custom foaming is a process in which a skier's foot is sealed into an inner boot through an injection of polyurethane foam. While custom foaming was popular with skiers, it was a constant nightmare for ski shop technicians who found the process messy, complicated and costly since any boot foamed improperly had to be thrown away. Two years after its introduction, the process was abandoned in favor of more economical inserts.
The inner boot is the lining underneath the shell. The lining is made of a pliable, cushioning material that provides insulation and added fit. This material can be a gel or synthetic that molds itself to the foot over extended use or it can be a material that, when heated, provides an instantaneous custom fit.